Magnetic resonance imaging is an imaging technique, well known in every respect, by which position encoding is effected by way of a variable magnetic field gradient. For imaging purposes, a high-frequency impulse is applied to the object under investigation, in order to dislodge the spins from their unexcited positions. After an evolution phase, in which the spin system can be manipulated, and in particular location encoding is undertaken, the signal is then detected. While a large-volume coil, the so-called body coil, is used for the purpose of excitation, for signal detection purposes a local coil array consisting or one or more sub-arrays is placed on the patient. Local coil arrays offer the advantage of greater sensitivity, i.e. their use permits a higher signal-to-noise ratio (SNR) to be achieved.
In the case of whole-body investigations the patient's head is, for example, placed in a head coil. This head coil has a plurality of individual coils, 90 and more coils have already been used in practice. Further coil arrays are arranged on the patient in the region of the thorax, the spinal column and the limbs. All the possible coils and coil arrays on the patient which are used for making measurements are then referred to as the local coil array.
These coils and coil arrays of the local coil array are arranged and attached on the patient outside the magnetic resonance system before the measurements start. For the purpose of carrying out a whole-body measurement, the patient is transported into the tunnel of the magnetic resonance system, but only a proportion of the coils is ever used. In order to reduce costs, there is only a short section within the magnetic resonance system which has an absolutely homogeneous magnetic field, referred to below as the homogeneous region. Measurements are only made in the homogeneous region, which has a length of about 30 cm. In the case of a whole-body investigation however, the coils cover a region of almost 2 m, which is why only a proportion of the coils is used at any one time for the measurements. So that cost savings can be made in the reception chain it is usual to provide only a limited number of receiving units, such as for example special preamplifiers, and to switch each of these to coils which are in the homogeneous region or close to the homogeneous region. The switching electronics which must be provided for this purpose are relatively complex and expensive.
A magnetic resonance system of this generic type is known from DE 10 2006 012 404 A1. There, it is proposed that a local coil element is attached to holders which can be moved and run along the tunnel wall, so that the local coil element can be moved from a storage position lying adjacent to the tunnel into a position on the patient. In another alternative, the local coil element is unrolled beneath the patient like a roller blind. The local coil element can also be stored in its extended state underneath the patient table.
DE 101 14 013 A1 relates to a magnetic resonance system in which a local coil is attached to a drivable carriage, so that it can be moved from a remote position onto the person, where the carriage is attached to a carriage holder which is arranged in a fixed location within the investigation space. In relation to the moving of the local coil onto the person, a fluid drive is disclosed which permits a translational displacement of the local coil from a position away from the person into a position close to the person.
DE 103 24 870 A1 discloses a magnetic resonance system with a local coil arranged in an investigation tunnel. This is arranged on floor-mounted swivel axes running parallel to the tunnel axis, and can be pivoted towards the patient.
DE 10 2006 046 287 A1 describes a field generation unit of a combined MR-PET system, in which use is made of a two-piece HF antenna arrangement, whereby a second part of the HF antenna arrangement, arranged on a horizontal board, can be held temporarily in a fixed position in the system. It is proposed that the second part is exchanged from one patient to another, and that second parts are provided which are pre-adapted for different patients.